Modulation



W. R. BLISS MODULATION Nov. 5, 1940.

Filed June 11, 19157' 5 Sheets-Sheet l NOV. 5, 1949. Wl R BLISSMODULATION Filed June ll; 1957 3 Sheets-Sheet 2 Nov. 5, 1940.

w. vFe. Buss MODULATION ZS'Sheets-Sheet 3 Filed June 11. 1957 E i iK.

Patented Nov. 5, 1940 UNITED STATES PATENT OFFICE MODULATION WilliamRoderic Bliss, Middletown, Conn.

Application June 11, 1937, Serial No. 147,688

Claims.

This invention pertains generally to modulation in electric circuits,and specifically to communication. The invention relates moreparticularly to the modulation of radio-frequency transmitters.

An object of the invention is to provide a new and improved modulator.

A further object is to improve upon the modulation of radio-frequencytransmitters.

Still another object is to provide a new and improved system for themodulation of a radiofrequency transmitter in a stage other than thefinal output-supplying, radio-frequency amplifier, without sacrificingefficiency in any stage.

Another object still is to provide a new and improved system forconverting amplitude into phase modulations and then to reconvert thephase modulation into amplitude.

Other and further objects will be explained hereinafter and will beparticularly pointed out in the appended claims.

In the drawings which accompany and form a part of the specification,Fig. 1 is a diagram illustrative of a circuit that may be used forintroducing modulation in accordance with the present invention; Fig. 2is a vector diagram illustrating the phase shift that may take place inthe amplifier occasioned by amplitude modulation in accordance with thisinvention; Fig. 3 is a diagram illustrative of anoutput circuit that maybe used according to the present invention; Fig. 4 is a vector diagramsimilar to Fig. 2 illustrating the relation that may exist between phaseangles and radio-frequency amplitudes in the output circuit; Fig. 5 is asimilar diagram illustrating the corresponding relation between phaseangles and radio-frequency amplitudes in the input circuit; Fig. 6 is adiagram similar to Fig. 3, illustrating an output circuit connected toutilize the modulation frequency present in the circuit for operating aloud-speaker or other suitable device; and Fig. 7 is a diagram of aradiofrequency transmitter employing the principlesA of this invention.

All terms used in this specification have meanings as defined in the1933 Report of the Standards Committee of The Institute of RadioEngineers. In accordance with conventional radiofrequency transmission,a source 2 of constantfrequency oscillations, shown in Fig. 7 as a.piezoelectric oscillatcr, may be coupled, through various stages, to anoutput circuit 4 that is, in turn, coupled to a radiating antenna 6.Three stages are illustrated: a first stage 8, a second stage l0. and athird stage I2. The modulating signal may be introduced into the systemin any desired way, as hereinafter described. It is customery tomodulate the output circuit 4, since'modulation of one of theintermediate stages 8, I0 and I2 introduces loss of efficiency.

In order to amplify without distortion a radio frequency modulated inamplitude, it is customary to employ a class B linear amplifier o rotherlowefficiency means of amplification. Class B amplifiers may havean efficiency as low as percent and they operate under smallerradio-frequency voltages in a more sensitive portion of thecharacteristic than is the case with class C amplifiers which, underhigh-efficiency conditions of operation, are inherently insensitive tosmall changes in either the grid circuit or the plate circuit, becauseof the relatively large voltage swing on the plate and the grid of thetube.

' According to the present invention,'an improved system is provided formodulating an intermediate stage without any sacrifice of efficiencywhatever. A small radio-frequency signal may be modulated in amplitude,in any desired conventional manner, sent through an amplitudedistortingamplifier, as in the stage I0, and it may be reproduced in the antenna 6or other load without distortion of the audio-frequency or otherlower-frequency modulation. The term audiofrequency will be employed inthe specification and claims in this broad sense, to denote anymodulation frequency. This may be effected, in accordance with a featureof the invention, with the aid of a high-efficiency class C amplifier,-and yet without producing distortion in the amplifier output circuit.Linear amplifiers may, however, from certain points of view, be employedwithout departing from the spirit and scope of the invention, as definedin the appended claims.

According to the preferred embodiment of the invention, the stage 8 isillustrated as comprising two vacuum-tube amplifiers 20 and 22containing radio-frequency oscillations, the respective input circuits24 and 26 of whichy are respectively coupled at 28 and 38 to the outputcircuit of the piezo-electric oscillator 2.

The piezo-electric crystal 34 is shown, for illustrative purposesmerely, as connected in the input circuit of the oscillator 2. The tunedoutput circuits 36 and 3 8 of the ampliers 20 and 22, which are thussupplied with a radio-frequency voltage of substantially the sameconstant frequency, are both shown coupled at 40 and 42 tothe inputcircuits 44 and 46 of two high-efiiciency class C distorting' amplifiers48 and 50, so as to supply substantially equal voltages to the grids 58and 60 thereof. 'I'he description may be more easily understood byreference to Fig. 1, in which are reproduced, among other things, theoutput circuits 36 and 38 of the amplifiers 20 and 22 and the inputcircuits 44 and 46 of the amplifiers 48 and 50, but omitting, forclearness, many of the elements shown in Fig. 7.

Though the voltages of the amplitude-modulated output circuits 36 and 38are of the same constant frequency, they always have phase differencesof substantially 90 degrees. The sum of their outputs in the. circuits44 and 46 has little amplitude variation, but is correspondinglyphasemodulated, the phase modulation or variation being dependent uponthe amplitude variations in the circuits 36-62 and 38`|0. In thecircuits 36 and 38, there is an amplitude shift, but no phase shift.

The desired phase difference may be produced in any desired way, as willbe understood from what follows. One or more phase adjusters may, ifdesired, be introduced into the coupling elements 28 and 30; aphase-adjusting condenser 52 is shown, by way of illustration, in thecoupling element 30. As it is only the phase difference between thevoltages as finally applied to the grids of the tubes 48 and 50 that isof importance, the circuit element 28 is shown unprovided with a phaseadjuster. The desired phase difference of 90 degrees may be attained byadjusting the condenser 52. The phase difference may be produced in anyone or more of the circuits 28, 30, 24, 26, 36, 38, 44 and 46 bysuitable tuning adjustment.

The coupling element 40 couples the output circuit 36 of the amplifier20 directly to a tuned circuit 62, and the coupling element 42 couplesthe output circuit 38 of the amplifier 22 directly to a tuned circuit64. These tuned circuits 62 and 64, respectively comprising a coil 66and a tuning condenser 68, and a coil '|0 and a tuning condenser '12,are each connected in both input circuits 44 and 46 of the amplifiers 48and 50. 'I'he connections of the tuned circuit 64 in the input circuitof the amplifier 50 may be traced from the cathode 'I4 of the amplifier50, which cathode 14 is grounded at 16, and through a biasing battery18, to the tuned circuit 64; from the tuned circuit 64, by way of aconductor 80, to a midpoint tap 82 of the coil 66; and from the midpointtap 82, through the lower half of the coil 66, and by way of a conductor84, to the grid of -the amplifier 50. The connections of the tunedcircuit 64 in the input circuit of the amplifier 48 may similarly betraced from the cathode 86 of the amplifier 48, which is similarlygrounded at 88, through the same biasing battery 18 and tuned circuit64, and by way of the same conductor 80, to the same mid-point 82; andthence, through the upper half of the coil 66, by way of a conductor 80,to the grid 58 of the amplifier 48.

Though the tuned circuit 64 is thus connected in both the input circuit46 of the amplifier 50 and the input circuit 44 of the amplifier 48, itwill be noted that the lower half of the coil 66 is con- `nected inseries therewith in the one case, and

Through their output circuits 36 and 38, therefore, the amplifiers 20and 22 impress upon each of the control grids 58 and 60 of theamplifiers 48 and 50 radio-frequency voltages of substantially the sameconstant frequency, but having a phase difference, the size of which isdependent upon the relative amplitudes of the voltages from theamplifiers 20 and 22 applied to these grids.

The amplitude modulation produced by the signal voltages is applied tothe amplifiers 20 and 22. The vector sum of the modulated radiofrequencies transmitted through the coupling links 40 and 42 on thegrids 58 and 60 is maintained constant. A phase modulation, but noamplitude shift, appears on the grids 58 and 60.

The connections of the amplifiers 20 and 22 to the grids 58 and 60 are,however, such that the voltage amplified by the amplifier 22 is appliedto these grids with phase unaffected, while the voltage of the amplifier20 is applied to the grids with phase reversed 180 degrees.

The amplifiers 48 and 50 are shown respectively provided with screengrids 54 and 56. This is for the purpose only of simplifying the circuitdiagram of Fig. '7. Any other type of amplifier tube may equally well beemployed in any stage, but it might then be necessary to introduceneutralization.

The amplifiers 20 and 22 are modulated in amplitude by a source 92 ofaudio power. The modulation of one of the amplifiers 20 and 22 is 180degrees out of phase with the modulation of the other amplifier. Thismay be effected in any desired way, as illustrated, for example, in Fig.7. The modulating signal may be introduced through the medium of anamplifier 94 which may, if

desired, be excited by a piezo-electric-crystal microphone 202. Theoutput circuit 86 of the microphone system is shown coupled, through atransformer 88, to two vacuum tubes |00 and |02, connectedtogether, inpush-pull. relation, with a half |04 of the secondary winding of thetransformer 98 connected in the input circuit of the tube |00 and theother half |06 in the input circuit of the tube |02. The output circuitsof the tubes |00 and |02 are respectively provided with windings |08 and||0 that are properly connected together in series, and that arecoupled, through condensers ||2 and ||4, to suitable series-connectedresistors ||6 and ||8. Other impedances than resistors may be employed,but resistors are preferred because the direct-current component ofthecurrent in the hereinafter-mentioned tube |40 is needed for modulation,due to the unsymmetrical output of the tube.

The resistor ||6 is connected to the output circuit 38 of the amplifier22, and the resistor ||8 to the output circuit 36 of the amplifier 20.The connections for the amplifier 22 are from the cathode |20 of theamplifier 22, which is grounded at |22, through the plate battery |24and the resistor ||6, and by way of a conductor to the output circuit38; and thence, by way of a conductor |26, to the plate |28 of theamplifier 22. The connections for the amplifier 20 are from the cathode|30, which is grounded at |32, and through the same battery |24 and theresistor |8, and by way of a conductor |34, to the output circuit 36;and thence, by way of a conductor |86, to the plate |38 of the tube 20.In this manner, two modulators are providedythrough the means of whichby suitable impedances and connections to the impedances, the modulatingsignal will be applied to the plates |28 and |38 one hundred and eightydegrees out of phase with each other.

The phase relations will be understood from Fig. 2, where Ea representsthe voltage from the tuned circuit 64 on the grids 58 and 68 of thetubes 48 and 58, and E1 and E2 represent the voltages, differing bydegrees in phase, on the respective grids from the tuned circuit 62. Thevector sum of E1 and E3 is E4. If E4 represents the voltage on, say, thegrid 58 of the tube 48, E5, the vector sum of E3 and E2, will thenrepresent the voltage on the grid 68 of the tube 58. The difference ofphase, represented by the angle a, between the voltage E4 or E5 oneither grid and the voltage E3 depends upon the relative amplitudes ofE1, Ez and E3. As E1 is always equal to E2, but opposite in sign, thedifference in phase between E4 and E3, and between E5 and E3, is thesame but in the opposite direction. Since the amplier 28, supplying thevoltages E1 and E2, and the amplier 22, supplying the voltage Es, aremodulated in amplitude, as above described, the difference in phasebetween the voltages E4 and E5 will change with the amplitude and thefrequency of modulation. In ordinary forms of modulation, obviously,this means that the phase difference can vary between 0 degrees-and 180degrees.

If the tubes 48 and 58 are class C amplitudedistorting amplifiers, it isdesirable to keep constant the radio-frequency amplitude on theirrespective grids 58 and 68. This condition may be obtained in anypreferred way, as by the use of an amplitude-detecting device,comprising the said tube |48, which is back-coupled to the modulator ofthe amplifier 28 in such a way as to oppose any change of amplitude onthe grids 58 and 68 of the tubes 48 and 58 by modulation of theamplifier 28. One scheme for accomplishing this back-coupled modulationis illustrated in the drawings. The plate coupling circuit of thedetector |48 extends from the cathode |46 of the tube |48, which isgrounded at |58, through the battery |24 and the resistor ||8, to theplate |48 of the detector |40. The tube |48 is shown provided with abiasing battery |42 by means of which it is biased so as to operate as adetector,

vand its grid |44 is coupled to the grid 68 of the tube 58 through acondenser 284. The voltage on the plate |48 of the tube |48 is therebycontrolled by the amplitude of the radio-frequency voltage on its grid|44, and this voltage is directly applied to the modulating voltage forthe amplifier 28. Since the tube |48 is biased negatively, an increasein radio-frequency amplitude on its grid |44 will cause the mean gridpotential to rise.

This, in turn, will cause the mean plate current, as averaged over anyradio-frequency cycle, to increase. When this current increases, the IRdrop through the plate resistor I8 increases, lowering the voltage onthe plate |48 of the tube |48, and hence on the plate |38 of the tube28. This decreases the push-pull voltage component on the grids 58 and68 of the tubes 48 and 58 and, through condenser 284, on the grid |44 ofthe tube |48. Analysis shows that voltage-amplitude fluctuation on thegrids 58 and 68 of the tubes 48 and 58 is thereby divided by- (l-l-p.)Where y represents the effective amplification of the tube 48. The sizeof ,u may be made large by proper choice of the circuit components andthe tubes, reducing the voltage-amplitude fluctuation on the grids 58and 68 of the tubes 48 and 58, and hence the distortion, to a negligiblequantity. The tube |48 is unaffected by phase shifts in itsradio-frequency grid voltage, and operates only upon amplitudefluctuations The amplitude-detecting device schematically represented bythe tube |48 may, of course, be constituted of several tubes in parallelor cascade, so connected as to increase the effective power oramplification of the back-coupled detector system.

Fig. 5 represents the relation between the phase angle a and themodulation of voltage V3 supplied by the tuned circuit 64 to the grids58 and 68 of the tubes 48 and 58. V4 and V5 represent the voltages,reversed in phase, supplied to the grids 58 and 68 by the tuned circuit62, as before described. If tube |48 were inoperative, the envelope ofall the vectors representing the radio frequency voltage on one of thegrids 58 or 88 would be the triangle ABD. That is, during the modulationprocess, the radio-frequency amplitude at any time during the-modulationcycle may be represented by a vector drawn from D to a point on the lineAB. V1, Vs, and V9 are examples of such voltages. On the other grid, 68or 58,' is a voltage represented by a vector Within the envelope ACD,which may be drawn symmetrical to the rst vector about the line AD. IfV1 is the voltage on, say, grid 58, V2 is the voltage at the same timeon grid 68. The phase difference is represented by the angle a betweenthe vector and the line AD.

In order to keep the length of all these vectors a constant, and thevoltage they represent unkvarying, the envelope should not be atriangle,

such as ABD, but a circular sector, such as AFD. The tube |48 producesthis change by increasing or decreasing the voltage on the amplifier 28;or, in other words, by varying the horizontal component of the vectorsof Fig. 5. Thus, for ex ample, if one radio-frequency amplitude Va bepresent at one instant,'the tube |48 will supply a voltageV7 to increasethe length of Vs to Vs, which is correct. Likewise, if a voltage V9appears, which is too large, the tube |48 will decrease it by an amountV10 to the size V11. This correction changes the phase angle of thevectox` to the correct angle for faithful reproduction of the modulatedvoltage V3 as P3 in the output circuit, as indicated in Fig. 4.

With no modulation, the voltage V3 should be half of its peak value;hence, the amplitude of the radio-frequency voltages delivered yto lthetuned circuits 62 and 64 should beso adjusted4 that a is 60 degrees, orthe totalphase difference between the voltage on the grids 58 and 68 ofthe tubes 48 and 58 is 120 degrees when there is no modulation of theamplifiers 28 and 22. The phase angle 2a may thus vary between zero and180 degrees. The absolute value of the vector V3 of Fig. 5, dividedbythe absolute value of the vector V1, is the cosine of half this phaseangle 2a. Since the absolute value of the vector V1 is the radius of.the circle, a constant, the absolute value of the vector V3 isproportional to the cosine of the angle a. The cosine of half the phaseangle `21:: of the resultant voltages applied to the respectiveamplifiers 48 and 58 is thus maintained substantially proportional tothe modulating voltage produced by one of the modulators. The factor ofproportionality, that is, the radius of the circle, is of no materialitybecause Fig. 5 is but a transition stage' to the vector diagram of Fig.4,

which represents conditions in the output cir.

justed as to hold constant the voltage of an amplitude equal to line DAof Fig. 5. Since arc EAF has a radius equal to DA, the vectors V1 andV2, drawn to the points where the arc EAF meets the sides AB and DC ofthe triangles DAB and DAC, need no correction. For all angles greaterthan the angle a of' the vector V1, the amplitude must be decreased, andfor all less than the angle a of the vector V1, they must be increased,except l when a=0, which is correct. It will be understood that theparticular size-ratio of triangle to arc chosen in Fig. 5 is forconvenience only. Other ratios may be used, if desired. In n o case maythe arc radius be larger, however, than DA of the triangle.

The amplifier need not be modulated at all by audio from the tube |02.All of its modulation may be obtained from the detector tube |40. Itsmodulation must consist of merely the correct 20 changes of amplitude tokeep the radio-frequency level on the grids 58 and 60 constant.Modulating with audio 180 degrees out of phase Will give a roughapproximation oi the required amplitude change, and the tube |40corrects for small deficlencies thereby produced. The invention is not,therefore, dependent for its operation upon reversed-phase signalvoltages.

A separate amplifying channel must be used, following each of the tubes48 and 50. These amplifying channels are shown at |52 and 54 in thestage I2, and they may comprise the ordinary high-efciency class C typeof amplier. The input circuit of the amplifier |52 is shown connected tothe output circuit of the amplifier 5 48, and the input circuit of theamplifier |54 is shown connected to the output circuit of the amplier50. The only substantial difference in radio-frequency Voltage betweenthe two channels at corresponding points is the phase angle beforedescribed. The selectivity of an ordinary class C amplifier is broadenough to permit lowfrequency changes of phase to take place Without anydistortion being occasioned by this phase shift. The present inventionpermits the use of class C amplifiers after modulation and of class Cradio-frequency amplifiers for audio amplification. Since, as beforestated, a class C amplifier is insensitive to small changes of amplitudein both the grid and plate circuits, a further advantage of the presentinvention is the reduction of noise which may be occasioned by the powersupplies to the amplifier tubes. 'Ihe invention further provides forhigh-plate power efficiency and filament power eiciency throughout theamplier.

'Ihe output circuit 4 of the transmitter is shown comprising twoshielded-grid tubes |56 and |58l connected ina differential relation,which becomes a push-pull relation when the phase dif- 60 ference is 180degrees. The output circuit of the amplifier |52 is coupled through avariable condenser |60 to the control grid |62 of the tube |56, and theoutput circuit of the amplifier |54 is similarly connected through avariable condenser |64 65 to the control grid |66 of the tube |58. |62and |66 are respectively connected, through resistors or otherimpedances |68 and |10, to the cathodes |12 and |14 of the respectivetubes |56 and 58. These cathodes |12 and |14 are con- .70 nected, by Wayof a grounded conductor |16, through a battery |18, and by way of aconductor |80, to one side of a tuned circuit |82. The other side of thetuned circuit |82 is connected, by way of a conductor |84, to themidpoint |86 of a coil |88 the ends of which are connected to the plates|92 and 94 of the respective tubes |56 and |58. A tuning condenser |96is connected in shunt to the coil |88. These output connectionsv areshown in a simplified form in Fig. 3.

The radio-frequency voltage appearing upon 5 the plates |92 and |94 ofthe tubes |56 and |58 is substantially free of amplitude variations, butthe phase relation of the voltages on these two plates is the same asthe relation in phase between the voltages on the grids 58 and 60 of the10 tubes 48 and 50. The tuned circuit |88, |96 will form a highimpedance to all components of voltage on the plates |92 and |94 of thetubes |56 and |58 whichare 180 degrees out of phase. The tuned circuit|82 pres-ents an impedance to all of 15 the components ofradio-frequency voltage from the tubes |56 and |58 which have nodifference in phase.

The amplifier 22 and the amplifiers 50 and |54 and related circuits maybe shielded at 200 from 20 the rest of the transmitter.

In Fig. 4, which is much like Fig. 5, P4 and P5 represent theradio-frequency voltage present in 4 the tuned circuit |88, |96. P3 mayrepresent the radio-frequency voltage appearing across the 25 tunedcircuit |82. In the input circuit of Fig. 1, the amplitude of thevoltage V3 was described as controlled by the modulating voltage. In theoutput circuit, the voltage V3 will appear as P3 in the same proportion,but amplified by the class 30 C amplifier channels |52 and |54. 'I'heuseful voltage developed across the tuned circuit |82 is thereforeequivalent to a large radio-frequency voltage amplitude modulated by alarge audiofrequency or other modulating-frequency voltage. To the tunedcircuit |82 a load |98 (Fig. 3), such as the antenna 6 of Fig. 7, may becoupled. The whole output circuit, comprising the circuits |82, |98, |88and |96, may thus be characterized as a phase-to-amplitude modulationconverter. 4o

If the tuned circuit |82 is replaced by an audiofrequency transformer,or other means of supplying the audio-frequency component of current tothe tubes |56 and |58, the whole amplifier may be used as a highefficiency audio amplifier. 45

Fig. 6 represents the output circuit of Flg. 3 so connected. In place ofthe tuned circuit |82 and the load |98, there is illustrated an audio orother modulating-frequency transformer 206, the secondary of which isconnected to a loud-speaker 60 208 or other means of using theaudio-frequency power. When the radio-frequency voltages in the tubes|56 and |58 are in phase, the tubes will draw a great deal of currentthrough the transformer 206. When the radio-frequency voltages 3 on theplates of the tubes 56 and |58 are 180 degrees out of phase, the tubeswill draw very little current. This phase and current fluctuation takeplace at an audio rate as controlled by the original modulation in thelow-power stage 8. The Go current change through the transformer 206supplies the audio power desired.

In practice, a suitable filter (not shown) may be inserted in the lead|84 for by-passing the radio-frequency component. if

The invention has many uses. Among the advantages of using this systemof modulation for radio-frequency transmitters are:

1. The elimination of high-power modulating equipment in largetransmitters;

2. The improvement in audio-frequency response characteristic of thetransmitter by elimination of losses invariably encountered in largetransformers and chokes;

3. The improvements in plate-circuit eiiiciency 75 CTI and lament-powerefficiency over any other method now known for amplifying a modulatedradio frequency without distortion;

4. The reduction of noise caused by power supplies through the use ofclass C amplifiers throughout the radio-frequency channels; and

5. 'I'he ease of adjustment. The class C radiofrequency amplifiers arenot critical with respect to plate, grid, or excitation voltages.

Due to the inherently wide modulated fre- .quency response of class Camplifiers, the present invention is extremely well adapted also totelevision transmitters.

The present invention provides a new and improved greatly simplifiedsystem for converting and reconverting phase and amplitude modulation,employing a plurality of modulated amplifiers 20 and 22, without theintroduction of unfavorable complex power factors, critical adjustmentsor losses, and without admitting amplitude changes.

Further modifications will occur to persons skilled in the art, and allsuch are considered to fall within the spirit and scope of theinvention.

What is claimed is:

1. An electric system comprising two circuits containing radio-frequencyoscillations of substantially the same frequency but differing in phase'by substantially 90 degrees, means for modulating the amplitude yof theoscillations of the two circuits, two amplifiers, means for applying toboth ampliers the voltage of one of the circuits unaffected in phase,means for applying to one of the amplifiers the voltage of the othercircuit unaffected in phase, means for applying to the other amplifierthe voltage of the other circuit but substantially reversed in phase, anoutput circuit coupled to the amplifiers, and means for preventingdistortion of the amplitude modulation of the radio-frequency voltage inthe output circuit.

2. An electric system comprising two radio-frequency circuits containingoscillations of substantially the same radio frequency but differing inphase by substantially 90 degrees, means for modulating the amplitude ofthe oscillations of the voltage of the other of the said two circuitsbut substantially reversed in phase, means for maintaining theradio-frequency amplitude applied to the amplifiers substantiallyconstant, a phase-to-amplitude-converting output circuit coupled to theamplifiers, means for reconverting the phase modulations into amplitudemodulations in the output circuit, and means for assuring linearity ofphase to amplitude conversion in the output circuit. y

3. An electric system comprising two radio-frequency circuits containingoscillations of substantially the same radio frequency .but differing inphase by substantially degrees, two amplifiers, means for applying toboth amplifiers the Voltage of one of the said two circuits unaffectedin phase, means for applying to one of the amplifiers the voltage of theother of the said two circuits unaected in phase, means for applying tothe other amplifier thavoltage of the other of the said two circuitsbutsubstantially reversed in phase, two modulators,-means comprising oneo f the modulators for vapplying signal voltages to one of the saidcircuits, means comprising the other modulator for applying the signalvoltages substantially reversed in phase to the other of the saidcircuits, an amplitude detector, and means for coupling the amplitudedetector to one of the said other modulator to oppose any tendency formodulation in one of the said circuits to introduce any changes ofamplitude in the amplifiers.

4. A radio-frequency transmitter comprising a substantiallyconstant-frequency oscillator, two

circuits, means for applying to the said two circuits the voltage of theoscillations of the constant-frequency` oscillator but differing inphase by substantially 90 degrees, means for modulating the amplitude ofthe oscillations in the said two circuits, two amplifiers, means forapplying to both amplifiers the voltage of one of the said two circuitsunaffected in phase, means for applying to one of the amplifiers thevoltage of the other of the said two circuits unaffected in phase, meansfor applying to the other ampli- -iier the voltage of the other of thesaid two circuits but substantially reversed in phase, means forAmaintaining the radio-frequency amplitude applied tothe amplifierssubstantially constant, an amplifying channel connected toeachamplifier, an output circuit to which the channels are connected, andmeans for reconverting the phase modulations into amplitude modulationsin the output circuit.

5. Anelectric system comprising two amplifiers each having an inputcircuit and an output circuit, means for impressing electricoscillations upon the amplifiers, two modulators for modulating theamplitude of the electric oscillations, an amplitude detector having aninput circuit and an output circuit, means for coupling the inputcircuit of the detector to the input circuits of the amplifiers, andmeans for coupling the output circuit of the detector to one only of themodulators.

6. An 'electric system vcomprising two first circuits, two secondcircuits for applying radio-frequency voltage to the first circuits,means for applying a voltage of predetermined frequency and phase to oneof .the second circuits, means for applying to the other second circuita voltage having the same frequency but differing in phase bysubstantially 90 degrees, means for applying one of the voltagesunaffected in phase to both rst circuits, means for applying the othervoltage to one of the first circuits unaect'ed in phase and to the otherfirst circuit substantially i reversed in phase, means for applying tothe respective second circuits signal voltages substantially reversed inphase, an outputcircuit coupled to the first circuits, and means formaintaining the voltage amplitude from the second circuits substantiallyconstant.

7. An electric system comprising a source of vradio-frequencyoscillations, means vfor` modulating the amplitude of the oscillations,an amplifier having an input circuit, means for impressing the modulatedelectric oscillations upon the amplifier, means for converting theamplitude modulations in the amplifier into phase modulations, an outputcircuit, means for delivering the phase modulations, to the outputcircuit, means for converting the phase modulations in the outputcircuit into amplitude modulations, and means for preventing distortionof the amplitude modulations in the output circuit comprisingamplitude-controlling means con- 1 nected between the modulating meansand the input circuit of the amplifier.

8. An electric system comprising a source of radio-frequencyoscillations, means for modulating the amplitude of the oscillations, alinear amplier, means for impressing the modulated electric oscillationsupon the amplifier, means for converting the amplitude modulations inthe amplifier into phase modulations, an output circuit, means fordelivering the phase modulations to the output circuit, meansfortconverting the phase modulations in theA output circuit intoamplitude modulations, and means for preventing distortion of theamplitude modulations in the output circuit comprisingamplitude-controlling means connected between the modulating means and aportion of the system that contains phase-modulated oscillations.

9. An electric system comprising two radiofrequency circuits containingoscillations of substantially the same radio frequency but differentphase, means comprising one orl more modulators for producing modulatingvoltages to modulate the amplitude of the oscillations of the twocircuits, two amplifiers, means for applying to both amplifiers thevoltage of one of thesaid two circuits unaffected in phase, means forapplying to one lof the amplifiers the voltage of the other circuitunaiected in phase, means for applying to the other amplifier thevoltage of the said other circuit but substantially reversed in phase.and means for maintaining the cosine oiV -modulated electricoscillations upon the ampliiiers, means for converting the amplitudemodulations inl the amplifiers into phase modulations, and means formaintaining the cosine of half the phase angleof the voltages of therespective amplifiers substantially proportional to the modulatingvoltage produced by one of the modulators.

W. RODERIC BLISS.

